H2O
carbonyl
K+ ion
Bi/CNS 150 Lecture 1
Monday, September 30, 2013
The ionic basis of neuroscience;
Introduction to the course.
Henry Lester
1
Who are the Bi/CNS 150 students?
Preliminary numbers
Total undergraduate enrollment, 38
12 seniors, 20 juniors, 5 sophomores, 1 freshman
Majors:
20 Biology,
3 CNS,
6 BE,
2 Ch,
3 ChE
2 Ph
2 CS
3 graduate students
Fields:
2 Bi
1 CNS
4 CCE
1 BE
1 ME
2
What is the most abundant molecule in an organism?
Molecule
Class Vote
Comments
water
3
Water is the most abundant molecule in an organism
H2O MW = 18
Density ~ 1 kg/l
Therefore the concentration of water in an aqueous solution is
~ (1000 g/liter )/(18 g/mol) = 55 mol/liter or 55 M.
All other molecules in the body are at least 100 times less concentrated.
Therefore we need to understand the properties of water.
4
Typical extracellular and cytosolic ion concentrations (mammalian cell)
Na+
major
monovalent K+
Ions
Cl-
divalent
cations
Other ions
Extracellular Intracellular
conc
(Cytosol)
145 mM
15 mM
4 mM
150 mM
110 mM
10 mM
Ca2+
2 mM
10-8 M
Mg2+
2 mM
0.5 mM
Pi-2
2 mM
40 mM
H+
10-7 M
10-7 M
Protein
0.2 mM
4 mM
5
One clue to a cell’s ionic concentrations:
Sea Water
Na+
major
monovalent K+
Ions
Cl-
divalent
cations
Other ions
Sea Water Extracellular Intracellular
conc
(Cytosol)
457 mM
145 mM
15 mM
9.7 mM
4 mM
150 mM
536 mM
110 mM
10 mM
Ca2+
10 mM
2 mM
10-8 M
Mg2+
56 mM
2 mM
0.5 mM
Pi-2
0.7 mM
2 mM
40 mM
10-7 M
10-7 M
10-7 M
0.2 mM
4 mM
H+
Protein
6
Membranes provide a barrier to diffusion around cells,
forming compartments
nicotine
Alberts 4th 2-22
© Garland
Alberts 4th 11-1
© Garland
. . . But specialized proteins
(channels and transporters)
control the permeation of
many molecules
natural or synthetic
lipid bilayer
Little Alberts 12-1
© Garland
7
A Cell that Lacks Concentration Gradients
External
Monovalent cations:
High Na+
Low K+
Na+
Na+
Na+ +
Na
Internal:
same as
External
Na+
Na+
Na+
Na+
Na+
K+
Na+
Na+
Na+
Na+
K+
8
Storing energy in a concentration gradient
without osmotic stress:
Simply reverse the ratio of Na+ and K+
External
Monovalent cations:
High Na+
Low K+
Na+
Na+
K+
K+
Internal:
Low Na+
High K+
Na+
Na+
Na+
Na+
K+
K+
K+
Na+
Na+
K+
9
The “Na+ pump” splits ATP to make a Na+ and K+ concentration gradient
3
Alberts 4th 11-8 © Garland
2
Alberts 4th 11-8 © Garland
From Kandel 6-5
10
Converting a concentration gradient
to an electrical potential:
Create permeability to one ionic species (K+)
Na+
Na+
Na+
K+
Na+
Lost positive charge
leads to net negative
interior potential
K+
K+
K+
K+
Na+
Na+
K+ channels
Na+
Na+
11
The Nernst potential:
the energy of discharging the concentration gradient for K+ ions
balances
the energy of moving the K+ ions through the potential difference
K+
K+
K+
K+
K+
12
Hundreds or thousands of ions
flow through a channel protein
for each opening
A transporter (or pump)
protein moves a few ions for
each conformational change
Kandel 5-19
13
Chem 1 textbook (OGC)
Figure 12-10
14
Deriving the Nernst potential (chemistry units)
G  RT ln
Ki
 zFV ; at equilibrium G  0 ; therefore
Ko
V 
RT  K i 

ln
zF  K o 
(we’ll assume that z = +1)
An e-fold ratio of K+ concentration ( Ki  Ko )
therefore leads to a potential difference of 
RT
.
F
R = 1.99 cal/mol oK; T = 300o; F = 9.65 x 104 C/mol (C is abbrev for coulomb).
OGN Figure 7-7
1.99 cal
 300
RT
mol


Therefore
=
F
9.965104 C
mol
 6 103 cal/C.
Now, 1 cal = 4.18 J (J is the abbreviation for joule),
and 1 J = 1 V x 1 C.
Therefore
RT
= 6 103 cal/C 4.18 V  C cal  25 mV.
F
Thus an e-fold concentration ratio gives a -25 mV membrane potential.
15
Deriving the Nernst potential
(physics units)
R = Nk, where N is Avogadro’s number and k is Boltzmann’s constant;
And F = Ne, where e is the charge on the electron.
23 J
 300
RT kT 1.38  10



 25 mV
Therefore
19
F
e
1.6  10 C
(we are familiar with the statement that kT = 25 meV)
----------------------------------------------
And a 10-fold concentration ratio leads to a membrane potential of
ln 10
RT
 58 mV
F
16
What is the selective advantage . . .
that the membrane is permeable at rest to K+ rather than to Na+?
a small inward leak of Na+ would change the internal [Na+]
by fractionally more than
a small outward leakage of K+ would change internal [K+ ]
Na+
Na+
Na+
[K+]I = 140 mM; [Na+]I = 10 mM. A leak of 10 mM:
[Na+] would increase from ~ 10 mM to 20 mM, doubling
[Na+]I and causing a 17 mV change in the Nernst
potential.
But a similar outward leak in K+ would decrease [K+]i
from 140 mM to 130 mM, causing a < 2 mV change in
the Nernst potential for [K+].
Na+
Na+
Conclusion: cell function is more stable when the resting
permeability is to K+ .
Na+
Na+
Na+
17
What is the selective advantage . . .
that the membrane is permeable at rest to K+ rather than to Na+?
Conclusion: cell function is more stable when the resting
permeability is to K+ .
Na+
Na+
Na+
Indeed, there are many dozens of K+ channels
in the genome, but only ~ 10 Na+ channels.
K channels are metabolically “free” at rest.
Na+
Important, because the “Na/K pump” splits
~ 2/3 of the brain’s ATP.
Na+
Na+
Na+
Na+
18
Other monovalent ions
Under what circumstances do neurons use Cl- fluxes?
Apparently it’s not straightforward to make a permeability pathway that
distinguishes among anions using protein side chains. Therefore there is
no “anion pair” corresponding to K+ / Na+. Few cells use anions to set the
resting potential.
But most postsynaptic inhibitory channels do use anion (mainly Cl-) fluxes.
Could neurons utilize plasma membrane H+ fluxes?
Probably not.
There are not enough protons to make a bulk flow, required for robustly
maintaining the ion concentration gradients.
(but some very small organelles (~ 0.1 mm) and bacteria do indeed store
energy as H+ gradients).
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Divalent Cations
What is the selective advantage that cells maintain Ca2+ at such low levels?
Cells made a commitment, more than a billion yr ago, to use high-energy
phosphate bonds for energy storage.
Therefore cells contain a high internal phosphate concentration.
But Ca phosphate is insoluble near neutral pH.
Therefore cells cannot have appreciable concentration of Ca2+;
they typically maintain Ca2+ at < 10 –8 M.
What is the selective advantage that cells don’t use Mg2+ fluxes?
The answer derives from considering the atomic-scale structure of a K+ selective channel (next slide), which received the 2003 Nobel Chemistry Prize:
(A suitable molecular graphics program, such as Swiss-prot viewer,
must be installed on your computer)
http://www.its.caltech.edu/~lester/Bi-150/kcsa.pdb
20
In the “selectivity filter” of most K+ channels,
K+ ions lose their waters of hydration and are co-ordinated by backbone carbonyl groups
H2O
carbonyl
K+ ion
(Like Kandel Figure 5-15)
21
Atomic-scale structure of (bacterial) Na+ channels (2011, 2012)
shows that here, too, partial loss of water is important for permeation
(As in Kandel Figure 5-1, Na+ channels select with their side chains)
Views
from the
extracellular
solution
The entire water-like pathway
Views
from the
membrane
plane
Payandeh et al,
Nature 2011;
Zhang et al,
Nature 2012
PDB files
4EKW, 4DXW
22
Time required to exchange waters of hydration
Na+ , K+
1 ns
(~ 109/s)
Ca2+
5 ns
(2 x 108/s)
Mg2+
10 ms
(105/s)
Na+ , K+, and Ca2+ can flow through single
channels at rates > 1000-fold greater than Mg2+
As the most charge-dense cation, Mg2+ holds its
waters of hydration most tightly.
The “surface / volume” principle:
We know of several Mg2 transporters,
but Mg2+ channels apparently exist only in
mitochondria & bacteria.
Moomaw & Maguire, Physiologist, 2008
23
Indeed, Mg2+ remains in the NMDA receptor channel so long . . .
that it becomes a voltage-dependent blocker
. . . this is crucial for learning and memory
Zigmond et al. (Eds.)
Fundamental Neuroscience,
© Sinauer (1999)
24
Primary (ATP-coupled) vs secondary (ion-coupled) pumps / transporters
Kandel 6-5
25
Cells have evolved elaborate processes for pumping out intracellular
Na+ and Ca2+
These gradients can be used in two ways:
Next image
1. The gradients are used for uphill “exchange” to control the
concentrations of other small molecules.
2. Transient, local increases in intracellular Ca2+ and Na+
concentrations can now be used for signaling inside cells!
26
Ion-coupled transporters in the plasma membrane also
control the levels of neurotransmitters
Antidepressants
(“SSRIs” =
serotonin-selective
reuptake inhibitors):
Prozac, Zoloft, Paxil,
Celexa, Luvox
Drugs of abuse:
MDMA
Attention-deficit
disorder medications:
Trademarks:
Ritalin, Dexedrine,
Adderall,
Strattera (?)
material
that won’t
of abuse:
Presynaptic MarksDrugs
an exam
cocaine
terminals appear on
amphetamine
Na+-coupled
cell membrane
serotonin
transporter
Na+-coupled
cell membrane
dopamine
transporter
cytosol
NH3+
HO
outside
HO
N
H
HO
H2
C
C
H2
NH3+
27
The “alternating access” mechanism explains
both ATP-driven (primary) and ion-coupled (secondary) transport
Based on structure
(Ca2+ pump)
Based on biochemistry
28
3 classes of proteins that transport ions across membranes:
(transporter)
modified from
Alberts 4th 11-4
© Garland
Ion channels that flux
many ions per event
Ion-coupled
transporters
“Active” transporters
(pumps) that split ATP
These proteins have evolved in a natural—perhaps necessary--way to provide that
•
The resting potential arises via selective permeability to K+
This selective permeability also leads to the Nernst potential.
Transient breakdowns in membrane potential are used as nerve signals.
•
Neuronal and non-neuronal cells also signal via transient influxes of Na+ and Ca2+.
29
Transport proteins (transporters, pumps, and channels)
are 5% of the human genome . . .
~ 1250 genes
30
The Bi / CNS 150
Home Page
http://www.cns.caltech.edu/bi150/
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Come to class, please. Quizzes occur randomly,
During ~ 1/3 of the lectures,
And count for 10% of your grade.
Exams will cover material in the lectures and the required readings in Kandel.
Don’t consult previous problem sets or exams.
32
Coursera
https://www.coursera.org/#course/drugsandbrain
Drugs and the Brain
7 weeks of lectures
Partial overlap with Bi/CNS 150.
Extra credit for Bi/CNS 150 students (~ 1/3 grade)..
Credit will be assigned **after** we make the Bi/BNS 150 curve;
Therefore you won’t be penalized for not taking the MOOC.
You must complete all MOOC work by 19 December 2013
33
If you drop the course,
or if you register late,
please email Teagan
(in addition to the Registrar’s cards).
Also, if you want to change sections,
please email Teagan
34
Henry Lester’s office hours occur at an unusual
time today: 12:30 -1:15 PM.
At the usual place: Outside the Red Door
End of Lecture 1
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